This invention is generally directed to liquid developer compositions, especially liquid developers with superior transfer efficiency, especially when, for example, transferring a toned electrostatic image from a photoreceptor to a substrate such as paper. More specifically, the present invention is directed to liquid developer compositions comprised of an oil base, black or colored polymer particles, charge control additives, and a stabilizer/flocculation component. Thus, in one important embodiment of the present invention there are provided liquid ink compositions with excellent transfer efficiencies exceeding 80 percent (percent by weight of the ink composition developed on the photoreceptor and transferred, for example, to paper) or greater, which inks are comprised of an oil base such as, Isopar, dyed polymer particles, charge control additives, and stabilizers, such as poly(ethylene-co-vinyl acetate) copolymers thereby enabling colloidally stable ink particles with a diameter of from about 0.1 micron to about 1.0 micron, which inks can be prepared in situ at temperatures exceeding 65.degree. C. In a further embodiment of the present invention, there are provided liquid ink compositions comprised of an oil component of, for example, Isopar, dyed polymer particles containing black or colored dyes, such as cyan, magenta or yellow; a charge control additive; and for the primary purpose of permitting flocculation of the particles at ambient temperatures a stabilizer/flocculation component, such as a poly(ethylene-co-vinyl acetate) copolymer, which enables transfer efficiencies of greater than 80 percent when transferring an electrostatic image from a photoreceptor imaging member to paper or transparent film. The liquid inks of the present invention can be selected for the development of images in various processes, inclusive of xerographic processes, electrostatic printing, and facsimile systems; color proofing processes; and the process as illustrated in Savin British Patent Publication No. 2,169,416, published July 9, 1986, the disclosure of which is totally incorporated herein by reference.
Development of electrostatic latent images with liquid developer compositions comprised of, for example, a dispersion of pigments in a liquid hydrocarbon are known. In these methods, the electrostatic latent image, which is usually formulated on a photoconductive member, is transported through a bath of the aforementioned liquid developer. Contact with the liquid developer causes the charged pigment particles present therein to migrate through the liquid to the photoreceptor surface in the configuration of a charged image. Thereafter, the toned electrostatic image is then electrostatically transferred from the photoconductor surface to plain paper. The image transfer efficiency from the photoreceptor to the paper is typically about 50 to 60 percent since the phenomenon of ink splitting between the photoreceptor and the paper usually occurs. In this process, the excess liquid toner remaining on the photoreceptor has to be cleaned therefrom prior to the next image cycle, and the excess toner discarded. Also, as only about 50 to 60 percent of the liquid toner is transferred to paper, the optical density of the image is often unacceptably low. Further, liquid toners exhibiting transfer efficiencies greater than about 80 percent also provide dense black images of optical density greater than 1.2, thus minimizing the amount of toner that has to be discarded and rendering an effective cleaning of the photoreceptor surface. The thin film of residual developer remaining on the surface of the sheet is then evaporated within a relatively short time period, usually less than 5 seconds. Also, the marking pigment particles may be fixed to the sheet by heat, for example, in image configuration.
There are disclosed in U.S. Pat. No. 3,554,946 liquid developers for electrophotography comprised of a carrier liquid consisting of a hydrocarbon, negatively electrostatically charged toner particles dispersed in the carrier liquid, and a pigment therein such as carbon black, aniline black, prussian blue, phthalocyanine red, and cadmium yellow. In accordance with the teachings of this patent, a copolymer is coated on the surface of the pigment particles for the primary purpose of imparting a negative electrostatic charge to these particles. Other patents disclosing similar liquid developer compositions include U.S. Pat. Nos. 3,623,986; 3,625,897; 3,976,583; 4,081,391 and 3,900,412. In the U.S. Pat. No. 3,900,412, there is specifically disclosed a stable developer comprised of a polymer core with a steric barrier attached to the surface of the polymer selected. In column 15 of this patent, there are disclosed colored liquid developers by selecting pigments or dyes, and physically dispersing them by ball milling or high shear mixing. Attempts to obtain color liquid developer compositions by the ball milling process described have been unsuccessful, particularly with respect to obtaining developed images of acceptable optical density in that, for example, the desired size for the latex particles is from 0.2 to 0.3 micron in diameter; and with ball milling techniques it is very difficult to provide a dispersion of carbon black or other pigment particles much smaller in size than about 0.7 to about 0.8 micron. Consequently, the addition of carbon black pigment particles, for example to latex particles with a diameter of 0.2 to 0.3 micron, result after ball milling in relatively small latex particles residing on the surface of the pigment particles.
Additionally, there is described in U.S. Pat. No. 4,476,210, the disclosure of which is totally incorporated herein by reference, liquid developers containing an insulating liquid dispersion medium with marking particles therein, which particles are comprised of a thermoplastic resin core substantially insoluble in the dispersion, an amphipathic block or graft copolymeric stabilizer irreversibly chemically or physically anchored to the thermoplastic resin core, and a colored dye imbibed in the thermoplastic resin core. The history and evolution of liquid developers is provided in the U.S. Pat. No. 4,476,210, reference columns 1 and 2 thereof.
With further regard to the British Patent Publication No. 2,169,416, the disclosure of which is totally incorporated herein by reference, there are illustrated liquid developer compositions comprised of toner particles associated with a pigment dispersed in a nonpolar liquid, and wherein the toner particles are formulated with a plurality of fibers or tendrils from a thermoplastic polymer, and possess a charge of polarity opposite to the polarity of the latent image. These toners apparently permit in some instances excellent transfer efficiencies, however, they have been found to be difficult to prepare, for example, with the aforementioned process batch-to-batch products are dissimilar. Also, the time needed to prepare such particles is about 30 hours, which is uneconomical. Furthermore, since the preferred resin, for example Elvax II 5720, is Isopar insoluble, it entraps Isopar during the mechanical grinding process. Consequently, during the fusing step energy has to be expended in exiting Isopar from the resin as well as fixing it to the paper surface. Accordingly, this results in an energy inefficient liquid toner. Furthermore, the toner particles of the British publication are of from about 2 to 2 microns in size. Ideally, variation and control of the particle size is an important consideration in liquid toner technology. Also, in some instances, the resulting inks do not permit acceptable transfer of the image.
Furthermore, there is illustrated in copending application U.S. Ser. No. 846,164, the disclosure of which is totally incorporated herein by reference, stable black liquid developers comprised of an insulating liquid medium having dispersed therein black marking particles comprised of a thermoplastic resin core which is substantially insoluble in the dispersion medium, and chemically or physically anchored to the resin core an amphipathic block or graph copolymer steric stabilizer which is soluble in the dispersion medium; and wherein dyes comprised of a specific mixture are imbibed in the thermoplastic resin core with the mixture of dyes being dispersible at the molecular level, and therefore soluble in the thermoplastic resin core and insoluble in the dispersion medium. Furthermore, in copending application U.S. Pat. No. 4,762,764, the disclosure of which is totally incorporated herein by reference, there is illustrated dyed sterically stabilized polymer particle for incorporation into negatively charged electrophoretic liquid developers.
The aforementioned liquid developers contain dyed sterically stabilized polymer particles, which are designed primarily for printing onto dielectric papers rather than for accomplishing the development of images present on a photoreceptor, and subsequently transferring the image to plain paper. With these inks and other prior art inks, it has been indicated that the particles are colloidally stable, thus when these particles are in close proximity there is a tendency for them to separate because of repulsive interactions thereby permitting the inks to undesirably readily split between the photoreceptor surface and the substrate surface during the transfer step. The aforementioned problem is eliminated with the inks of the present application by rendering these inks flocculated, thus the forces between the ink particles are attractive rather than repulsive, and the resulting inks cannot be as readily split when being transferred to paper. Accordingly, with the inks of the present invention there are provided excellent transfer efficiencies, and acceptable edge acuity for the final images. Moreover, the aforementioned inks are prepared in a manner to enable the sterically stabilized polymer particles selected to possess stability when formed, which particles subsequently flocculate upon cooling to ambient temperature enabling images of high resolution and wherein the transfer efficiency exceeds, for example, 80 percent to substrates such as paper.
Other prior art of interest includes U.S. Pat. Nos. 4,454,215 and 4,582,774 directed to liquid developers, and more specifically suitable toner particles present in a specific liquid carrier so as to form readily disassociated flocs, reference column 4, lines 63 to 68, and continuing on to column 5, lines 1 to 14. Patents primarily of background interest are U.S. Pat. Nos. 4,306,009; 4,363,863; 4,374,918 and 4,521,505.
Although the above described liquid inks are suitable for their intended purposes, there remains a need for new liquid developers. More specifically, there is a need for liquid developers with superior transfer efficiencies. There also is a need for colored liquid developers which possess the aforementioned characteristics. Additionally, there is a need for economical liquid developer compositions that permit images of excellent resolution in a number of known imaging processes. Moreover, there is a need for liquid developers wherein the colorants selected are suitably dispersed such that the primary particles are submicron in size, preferably an average diameter of 0.2 to 8 microns, thus enabling black or colored images of excellent resolution. Further, there remains a need for liquid developers wherein there is included therein certain stabilizers that enable flocculation of the developer at ambient temperature.